YA book about kid assassins learning their craft and realizing they're the bad guys. Read around 2010-2014.

[Cristina]

i read a book a long time ago around 2010-2014ish and it has been bugging me that I forgot the title. It would be awesome if someone helped me find it! It may have had a sword and an airship on either the first and second book.

Its about some kid assassins learning their craft from their parents and their parents friends. They live kind of in the forest(?) together and have an enclave-like village.

They can step into an empty space, to either retrieve items or store them there for their missions. If you spend too much time in that dimension, you begin to freeze and just forget yourself, so they have to recite a mantra to stay grounded.

I think they turn on their parents or one of their parents' friends because he's been storing people in that magical dimension to take over their order(??), might have been in the second book or end of first?

They finally get taken on a mission and realize that they're actually the bad guys. They watch their parents slaughter a whole family. In the second book, one of them falls in love with the other and then goes to an opium den to forget his feelings? That's all I can remember.

tldr:

Twin/dual protagonists: Two young assassin trainees, they're cousins or grow up believing they're related but actually aren't since their 'uncles' are just friends of their parents.

Pocket-dimension: A magical space used to store gear and people.

Side effect: Staying too long in the dimension causes freezing and memory loss—requiring a mantra to stay grounded.

Family betrayal: They witness their parents (and parent friends) murdering an innocent family on their first 'you are ready' mission

Coming-of-age twist: In the second book, one protagonist falls in love with the other.